Context. Short-period high-amplitude pulsating stars of Population I (δ Sct
stars) and II (SX Phe variables) exist in
the lower part of the classical (Cepheid) instability strip. Most of them
have very simple pulsational behaviours, only one or two radial
modes being excited. Nevertheless, BL Cam is a unique object among them, being an extreme metal-deficient field high-amplitude SX Phe variable with a large
number of frequencies. Based on a frequency analysis, a pulsational interpretation was previously given.

Aims. We attempt to interpret the long-term behaviour of the residuals that were not
taken into account in the previous Observed-Calculated (O–C) short-term analyses.

Methods. An investigation of the O–C times has been carried out,
using a data set based on the previous published times of light maxima,
largely enriched by those obtained during an intensive multisite
photometric campaign of BL Cam lasting several months.

Results. In addition to a positive (161 ± 3) × 10-9 yr-1 secular
relative increase in the main pulsation period of BL Cam, we detected in the O–C data short- (144.2 d) and long-term (~3400 d) variations, both incompatible with a
scenario of stellar evolution.

Conclusions. Interpreted as a light travel-time effect, the short-term O–C variation
is indicative of a massive stellar component (0.46 to 1 ) with a
short period orbit (144.2 d), within a distance of
0.7 AU from the primary. More observations are needed to confirm the long-term O–C variations: if they were also to be caused by a light travel-time effect, they could be interpreted in terms of a third component, in this case probably a
brown dwarf star (≥0.03 ), orbiting in ~3400 d at
a distance of 4.5 AU from the primary.

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